std: Fix some behavior without stdio handles

On all platforms, reading from stdin where the actual stdin isn't present should
return 0 bytes as having been read rather than the entire buffer.

On Windows, handle the case where we're inheriting stdio handles but one of them
isn't present. Currently the behavior is to fail returning an I/O error but
instead this commit corrects it to detecting this situation and propagating the
non-set handle.

Closes #31167

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstd/io/stdio.rs b/src/libstd/io/stdio.rs
index 79091fd3d6b..cd2d5e52462 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -112,7 +112,7 @@ impl<W: io::Write> io::Write for Maybe<W> {
 impl<R: io::Read> io::Read for Maybe<R> {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         match *self {
-            Maybe::Real(ref mut r) => handle_ebadf(r.read(buf), buf.len()),
+            Maybe::Real(ref mut r) => handle_ebadf(r.read(buf), 0),
             Maybe::Fake => Ok(0)
         }
     }